The present invention generally relates to step-up circuits, and more particularly, step-up circuits that perform a charge pump operation.
To drive an LCD panel, for example, a voltage of 12-18V may be required when the duty is {fraction (1/100)}. However, since the power supply voltage of recent Integrated Circuits (“ICs”) is a DC voltage of 1.8-3.6V, the power supply voltage must be stepped up by a step-up circuit to drive a LCD panel with such voltages.
FIG. 9(a) shows a state in which a step-up circuit is not operating. As indicated in FIG. 9(b), when clock signals V1-V4 are supplied to gates of transistors Q1-Q4, respectively, the step-up circuit starts its operation, and steps up a voltage between a first power supply potential VDD and a second power supply potential VSS to output an output potential VOUT.
In FIG. 9(b), the transistors Q2 and Q4 are turned on, a current flows in a direction indicated by an arrow, and a charge is supplied to a flying capacitor C1. In this instance, the power supply potential VDD drops momentarily. If the same power supply potential VDD is also used in other circuits that are sensitive to changes in the power supply potential, these circuits may possibly malfunction. Furthermore, when a plurality of step-up circuits like the one indicated in FIGS. 9(a) and 9(b) are used, a change in the power supply potential VDD tends to become greater.
Accordingly, there is a need for step-up circuits that can reduce and/or possibly eliminate changes in the power supply potential upon starting a step-up circuit.